Implement TDS cursor streaming (sp_cursoropen/sp_cursorfetch/sp_cursorclose) for Repo.stream support

[zerobatu]

Summary

Implements the DBConnection cursor callbacks (handle_declare/4, handle_fetch/4, handle_deallocate/4) in Tds.Protocol using SQL Server's cursor stored procedures, enabling Repo.stream/2 and DBConnection.stream/4 support.

Changes

Core Implementation

• lib/tds/protocol.ex: Replaced stub implementations of handle_declare/4, handle_fetch/4, and handle_deallocate/4 with working cursor operations:

  • handle_declare/4 uses sp_cursoropen (ProcID 2) with FORWARD_ONLY / READ_ONLY cursor options. Automatically enables PARAMETERIZED_STMT mode (scrollopt 0x1004) when parameters are present.
  • handle_fetch/4 uses sp_cursorfetch (ProcID 7) with NEXT fetch type and configurable max_rows.
  • handle_deallocate/4 uses sp_cursorclose (ProcID 9) to close the cursor.
  • Updated message(:executing, ...) to include return value parameters in Tds.Result so the cursor handle can be extracted from sp_cursoropen's output.

• lib/tds/messages.ex:

  • Uncommented cursor stored procedure IDs (@tds_sp_cursoropen, @tds_sp_cursorfetch, @tds_sp_cursorclose).
  • Added encode_rpc clauses for :sp_cursoropen, :sp_cursorfetch, :sp_cursorclose.
  • Added {:returnvalue, param} handling in parse(:executing, ...) to capture output parameters from cursor RPCs.

• lib/tds/result.ex: Added params field to Tds.Result struct for storing RPC output/return value parameters.

• lib/tds/tokens.ex: Added support for TABNAME (0xA4) and COLINFO (0xA5) tokens sent by SQL Server in cursor result sets.

• lib/tds.ex: Added Tds.stream/4 public API function delegating to DBConnection.stream/4.

Tests

• test/stream_test.exs: 12 tests covering:
  • Simple streaming queries
  • Chunked streaming with max_rows
  • Parameterized queries
  • Empty result sets
  • DBConnection.stream/4 direct usage
  • Lazy enumeration with Stream.map and Enum.take
  • Lazy batch processing with Stream.each and side effects (Agent)
  • Progressive transformation with Stream.flat_map
  • Streaming after a prior query on the same connection
  • Connection recovery after stream errors
  • Reusable prepared queries

Notes

• SQL Server cursors add a ROWSTAT column to result sets. Users should be aware of this additional column when processing stream results.
• Streaming requires an active transaction (SQL Server requirement).
• The :max_rows option controls fetch batch size and defaults to 500, matching Ecto.Repo.stream/2 convention.